An MR sensor detects magnetic field signals through the resistance changes of a magneto-resistive element, fabaricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. The conventional MR sensor operates on the basis of the anisotropic magneto-resistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in an MR head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage.
A different and more pronounced magneto-resistance, called a giant magneto-resistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the ferro-magnetic layers as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferro-magnetic layers is fixed or pinned. The physical origin is the same in all types of GMR structures; namely, the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferro-magnetic layers. This in turn causes a change in the spin-dependent scattering of conductive electrons and, thus, the electrical resistance of the structure. The resistance of the structure changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure including two essentially uncoupled ferro-magnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferro-magnetic layers in “pinned.” The pinning may be achieved by depositing the ferro-magnetic layer to be pinned onto an antiferro-magnetic layer, such as an iron-manganese (Fe—Mn) layer, to create an interfacial exchange coupling between the two layers. The spin structure of the antiferromagnetic layer can be aligned along a desired direction (in the plane of the layer) by heating beyond the “blocking” temperature of the antiferromagnetic layer and cooling in the presence of a magnetic field. The blocking temperature is the temperature at which exchange anisotropy vanishes because the local anisotropy of the antiferro-magnetic layer, which decreases with temperature, has become too small to anchor the antiferromagnetic spins to the crystallographic lattice. The unpinned or “free” ferromagnetic layer may also have the magnetization of its extensions (those portions of the free layer on either side of the central active sensing region) also fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the free-layer central active region is free to rotate in the presence of an external field. The magnetization in the free-layer extensions may be fixed by longitudinal hard biasing or exchange coupling to an antiferromagnetic layer. However, if exchange coupling is used, the antiferromagnetic material is different from the antiferromagnetic material used to pin the pinned layer and is typically nickel-manganese (Ni—Mn). This resulting structure is called a “spin valve” (SV) MR sensor.
The spin-valve head has the same stabilization issues as conventionally designed GMR heads. In particular, these problems are exacerbated by its multilayer structure. Typically, there are three films whose domain structures contribute directly to the sensitivity, signal-to-noise, and stability of the sensor. Namely, the antiferromagnetic (AF) pinning layer, the Co alloy pinned layer, and the NiFe free layer. With respect to conventional MR heads, the spin valve has an additional serious and unique reliability concern in that the AF/Co alloy structure is unstable and can be easily induced to rotate its magnetization. The sensor output is strongly influenced by the AF orientation and, as a result, the disturbed sensor may show poor asymmetry, degraded sensitivity, and increased noise compared to its performance in the intended orientation. The stability of the antiferromagnetic layer is the Achilles' heel of the spin valve. The misorientation of the AF magnetic pinning field can occur spontaneously or as a result of heating from electrical overstress, thermal asperities, or external influences. Heat, together with the magnetic field from the sense current, inverts the magnetization of the AF film. The creation of a thermally stable, antiferromagnetic film has become an important design criterion for spin valves. Thus, it is important to detect the pin layer reversal. This is especially true during servo operation.
In addition, during servo operation, it has been found that the polarity of the bits can be reversed. Thus, when reading data, it is important to detect the occurrences when this polarity has been reversed. For example, a pattern of bits or sync field are required to be identified. A bit reversal can make this identification difficult.
In conventional rotating disk data storage systems, it is common to employ some type of servo system to determine the radial position of the read/write transducer head over the disk surface and to maintain the transducer head over the center line of one of the concentric recording tracks during data reading and data writing operations. This is accomplished by providing servo information on one or more of the disk surfaces for access by the read/write transducer heads. In prior art, disk drives have included various known types of head positioning servo systems. In a pertinent prior art type of servo system, often referred to as an imbedded servo, the prerecorded servo information occupies positions (servo sectors) of each disk's recording surface, with the servo sectors being angularly spaced apart and interspersed among the data sectors of each concentric track. Servo sectors are prerecorded on the recording surface in arcuate sections, called frames, that run radially along the disk surface from the center to the outer edge. Frames are typically written at discrete angular intervals such that as the recording surface is rotated beneath an active read/write transducer head, servo sectors pass beneath the active head in time-quantifiable phases. Each servo phase represents the angular position of that servo frame on the recording surface, and the length of each servo phase defines a time period for servo processing circuitry in which servo information contained in the servo sector is presumed valid.
Furthermore, a servo sync field word includes a robust sync pattern which is written radially phase coherently by the servo writer as part of the manufacturing process such that the synchronization can always be achieved during a seek mode where an active read/write head may be positioned along the tracks.